n00b anycast question

the only thing I can think of is to make sure each provider will accept the advertisement. Often times they put an exact match on the subnet size of the advertisement, and will drop anything smaller. Make sure they are setup to accept the smaller subset of your ip block that you want to use for anycast.

I also believe you will need to use an entire /24 at a minimum for this. I don't think they will accept a smaller prefix. Even if they do, other routers on the internet may drop smaller advertisements, giving you mixed results.